This invention relates to a barometer of the type having a stabilized membrane capsule, a base plate supporting the membrane capsule, a pointer mechanism which is located on one side of the membrane capsule and which includes two substantially parallel plates, in which a pointer shaft is rotatably mounted, and a drive element.
Such a barometer is known and is called an aneroid barometer, the membrane capsule of which consists of two metal membranes joined at the edges in a gas-tight manner and is completely evacuated. A suitable prebiasing of the metal membranes ensures that the metal membranes are not pressed against one another by the ambient air pressure, but are spaced apart to a greater or lesser extent depending on the lever of the air pressure. Such a membrane capsule is designated as a stabilized membrane capsule as compared to unstabilized membrane capsules which are also known and in which the metal membranes are pulled apart by an additional support spring.
Since the one metal membrane is secured to the base plate, the other metal membrane executes an appropriate movement as the air pressure changes. This movement, called excursion, is transmitted by means of a transmission mechanism to a pointer on the pointer shaft whose position can be read off a dial. The transmission mechanism must transform the minute excursions of the membrane capsule into a large pointer movement and, for this purpose, has a large transmission ratio. The transmission mechanism in the known barometer includes a double armed lever which is pivotally articulated on the base plate and which has a long lever arm engaging the second lever arm of the lever shaft which is mounted on the base plate. A chain forming the drive element of the pointer mechanism is positioned about the pointer shaft and is secured or attached to the first lever arm of said lever shaft. The plate of the pointer mechanism which faces toward the membrane capsule includes a member which projects beyond the edge of the plate and which is attached to the base plate by means of a stud bolt.
In a barometer with an unstabilized membrane capsule which is known in the art, the transmission mechanism differs from the structure described hereinbefore in that in place of the pivotal double armed lever, a lever is secured to the support spring which engages the second lever arm of the lever shaft.
In addition to pointer mechanisms featuring a chain or a thread, pointer mechanisms are also known which comprise a pinion on the pointer shaft and a meshing toothed quadrant with which the first pointer arm of the pointer shaft is in engagement.
The drawback of the known barometer is that the transmission mechanism thereof requires considerable expenditure of manufacturing and assembly work and also takes up considerable space. The base plate, for instance, must have a distinctly larger diameter than the membrane capsule to enable the base plate to support the pivotal, double armed lever as well as the lever shaft.
Furthermore, the transmission mechanism, which consists of numerous individual elements, cannot be pre-fabricated as a unit, on the contrary, the pointer mechanism of the elements, the double armed lever and the lever shaft must be separately mounted on the base plate.
Another disadvantage of the known barometer is that, owing to the high frictional resistance in the transmission mechanism, minor variations in air pressure are not automatically indicated. In order to be able to detect minor variations in air pressure by means of this known barometer, one must tap the barometer with one's finger, for instance, thereby subjecting it to vibration and thus overcoming the resistance to movement which prevails at various locations in the transmission mechanism so that, only after such manipulation, will the barometer suddenly be adjusted to the new air pressure value.